Pressure transfer device serve to transfer the pressure of a measured medium via a hydraulic path to a pressure sensor. For this, a pressure transfer device or means usually includes a body of the pressure transfer means, on which, by means of an circumferential joint, is pressure-tightly secured an isolating diaphragm, forming a chamber of the pressure transfer means between the isolating diaphragm and the body of the pressure transfer means. From the chamber of the pressure transfer means, a hydraulic path extends through the body of the pressure transfer means to the pressure sensor, in order to supply this with a media pressure, which the isolating diaphragm is exposed to on its outer side facing away from the pressure transfer means.
The transfer liquid, most often an oil, enclosed in the hydraulic path and in the chamber of the pressure transfer means causes, due to its temperature-dependent volume expansion, a variable position of the isolating diaphragm. In such case, it is to be noted that established pressure sensors—for example, semiconductor sensors or capacitive sensors—have an extremely small pressure-dependent deflection of their measuring elements, so that, in the measuring range of the pressure measuring devices, at most negligible pressure-dependent volume displacements of the transfer liquid occur, and the position of the isolating diaphragm determined by the temperature describes well in first approximation its actual position for measurement operation.
Insofar as an isolating diaphragm is an elastic component, the deflection of the isolating diaphragm when shifting its position effects an additional pressure dp between the measured pressure px presiding on the outside of the isolating diaphragm and the chamber pressure pk in the chamber of the pressure transfer means. This pressure dp=px−pk can, especially in the case of small measuring ranges, far exceed the measurement error of the actual pressure sensor.
The absolute value of dp=px−pk is not actually of importance. For, if this value were constant, then it could easily be compensated for during a measuring. The problem lies in the fact that dp changes with the deflection of the isolating diaphragm, and that the size of the changes increases with the size of the absolute value of dp.
There are therefore numerous approaches to minimize the pressure dp.
In industrial process measurements technology, isolating diaphragms with an impressed wave pattern especially have become common. These diaphragms are, in the case of small deflections, stiffer than planar diaphragms; however, they enable, as a whole, larger deflections.
German Patent DE19946234C1 describes a diaphragm for a pressure transfer means, which has a circular central area and, on the edge, an edge surface for clamping, both of which are connected with one another via annular surfaces arranged concentrically to one another and offset from one another in axial direction in the form of steps, wherein the individual annular surfaces are alternately inclined radially outward and radially inward with respect to a plane parallel to the diaphragm plane, wherein each of the annular surfaces, rounded off with a radius, transitions via a step to the next annular area. This shaping of the isolating diaphragm is supposed to have the effect, that the movements of the isolating diaphragm caused by thermal expansions, thus the change of its equilibrium position, corresponds to the shifting of the working point of the isolating diaphragm caused by the volume expansion of the transfer liquid.
A similar approach is described in the Offenlegungsschrift DE10031120A1, according to which the coefficients of thermal expansion of an isolating diaphragm and the body of the pressure transfer means on which the isolating diaphragm is secured, as well as the coefficient of thermal expansion of the pressure transfer medium are matched to one another in such a manner, that a thermally caused volume change of the transfer liquid can be absorbed by a shifting of the resting position of the isolating diaphragm in the chamber of the pressure transfer means between the isolating diaphragm and the body of the pressure transfer means.
The two described approaches are theoretically very interesting; however, in practice, they are only feasible in a very limited manner, for they presuppose temperature equilibrium between the transfer liquid, the isolating diaphragm and the body of the pressure transfer means. This presupposition is, for most applications in process measurements technology, not fulfilled, so that, in the case of temperature jumps, the described arrangements can cause even larger measurement errors than pressure measuring arrangements with conventional isolating diaphragms without a temperature-dependent control function.
Another point of view concerning the pressure due to the isolating diaphragm deflection is considered in European patent EP1114987B1. In this document, the concern is not to cause the contribution of the isolating diaphragm to the pressure to disappear, but rather instead to linearize it. For this, a diaphragm is provided which has annular, trapezoidal surfaces, which, in each case, are connected with one another via inclines of 45°.
The linearizing of the pressure p(T) is based on the idea that, with knowledge of the temperature, a simple compensation for the pressure p(T) should be possible. The determining of the effective temperature, which defines the actual volume of the transfer liquid, is, however, in most cases, in which no temperature equilibrium is given, extremely complicated.
A completely different approach is described in Offenlegungsschrift DE102005023021A1, according to which a pressure transfer means has an isolating diaphragm having at least two equilibrium positions, and wherein the working point of the pressure transfer means for the temperature range in question should always lie between these two equilibrium positions. This means, however, that the isolating diaphragm is bistable, and can display an oil canning behavior, as discussed, for example, in DE 10152681A1
In DE 102005023021A1, it is described that the diaphragm is first embossed with a wave pattern, wherein the annular waves alternately have heights H1 and H2, and the diaphragm is then axially compressed with a plunger, wherein, through the compression, material is shifted radially from the waves with the greater height H1 to the waves with the smaller height H2. This should effect that the inner region of the isolating diaphragm can assume at least two different rest positions. Then, in the ideal case, there should be used with the liquid exactly that amount of pressure, with which essentially no return force acts in the direction of one of the two rest positions, since the diaphragm behaves at this working point as a restoring-forceless diaphragm.
This idea is interesting at a first glance; it overlooks, however, that this condition of restoring-forcelessness is only fulfilled exactly in the equilibrium positions, and that between the equilibrium positions, the diaphragm always seeks the most energetically favorable, nearest equilibrium position, which can have for the pressure p the effect of a hysteresis or oil canning behavior.
In the case of the prevalent variety of diameters and material thicknesses of isolating diaphragms having the most varied of dimensions and embossed patters, for comparison of performance, it is usual to use a dimensionless pressure p defined as follows:
                              p          :=                                    q              ·                              a                4                                                    E              ·                              h                4                                                    ,                            (        1        )            
In such case, q is the pressure earlier referred to with dp due to a deflection of the isolating diaphragm, h the diaphragm's material thickness, a its radius and E the modulus of elasticity of the material of the isolating diaphragm.
The dimensionless pressure p can especially be presented as a function of a dimensionless deflection w, wherein w is defined as
w:=y/h, wherein y is a length which is dependent on the volume change V of the transfer liquid, which has caused the deflection of the isolating diaphragm.
Helpful here is an estimation of y as the height of a cone, whose base equals the movable area of the isolating diaphragm and whose volume is equal to the volume change of V, thus:
                              w          :=                                    1              h                        ·                                          3                ·                V                                            π                ·                                  a                  2                                                                    ,                            (        2        )            
It should again expressly be emphasized that with the deflection y and the variable w derived therefrom need not be associated a directly measurable length on the isolating diaphragm. It is only a simple volume-proportional length measurement to provide a basis for the comparison of isolating diaphragms.
A comprehensive overview for the design of isolating diaphragms is given by the formalism for description of isolating diaphragms with an impressed sinusoidal wave pattern in “Flat and corrugated diaphragm design handbook” by Mario Di Giovanni, which builds upon the work of the Russians Feodos'ev and Andreeva. According to this, the following relationship holds:p:=A(q)·w+B(q)·w3  (3),wherein stiffness coefficients A and B are functions of a variable q, which, in turn, is a function of the height of the impressed wave pattern, thus: q2:=1.5*(H/h)2+1, wherein is the height of the embossed pattern. For other details of the functions A(q) and B(q), reference should be made to the work of Di Giovanni. With the increasing height of the embossed pattern, in any event, A becomes larger and B smaller. In a diaphragm design, it is thus to be established which maximum deflection wmax is provided, in order to keep the value p(wmax) as small as possible.
When, for example, an isolating diaphragm having a wave pattern for a deflection of up to a value of wmax=5 should be designed, this means a value for p of, for instance, 140. For a diaphragm with a radius a=29 mm, a material thickness of h=100 μm, and a modulus of elasticity of 200 GPa, this means a diaphragm error of, for instance, 40 mbar, wherein w, under the assumption of the above described approximation with a conical volume, corresponds, say, to a volume deflection of, for instance, 430 μl. Such volume deflections can, in the case of capillary pressure transmitting means, certainly occur, and a measurement error of 40 mbar is—especially in the case of small measuring ranges of, for example, 100 mbar—not acceptable. The fundamental limits of wave diaphragms are thereby clearly evident.
Diaphragms with a trapezoidal or step-shaped contour enable diaphragm designs, in the case of which the value for p(w) is, for instance, halved in comparison with comparable wave diaphragms. Even this is still not sufficient for many applications.